Microsoft has now responded with a post on the Vista team's blog, with info from Dave Marsh, a lead program manager responsible for Windows' handling of video. And the response is pretty much what you'd expect: Winodws already contains code to support content protection (eg for DVDs), the code in Vista is required to play copy-protected high-def content, and it generally doesn't have any impact in other areas.

There's a Top 20 questions, including this interesting example:

Will the Windows Vista content protection board robustness recommendations increase the cost of graphics cards and reduce the number of build options?

Everything was moving to be integrated on the one chip anyway and this is independent of content protection recommendations. Given that cost (particularly chip cost) is most heavily influenced by volume, it is actually better to avoid making things optional through the use of external chips. It is a happy side effect that this technology trend also reduces the number of vulnerable tracks on the board.

One of the problems is that most people don't have any experience of protected high-definition content on Vista (including me), and most people have no comparable experience with other platforms. We can't, for example, compare Microsoft's approach with what Apple is doing in OS X, because Apple is keeping it secret. (It seems unlikely that Macs will be unable to play protected HD content.)